Rare earth 123 superconductors (RE-123 superconductors) are excellent in magnetic field characteristics at liquid nitrogen temperature compared with Bi superconductors (Bi—Sr—Ca—Cu—O superconductors), therefore, it is possible to achieve a high critical current density (Jc) suitable for practical use under a high magnetic field. If a practical application of this wire material is successful, a production process without using silver, which is a noble metal, is possible as well as it has an excellent characteristic in a high temperature region, and liquid nitrogen can be used for a cooling medium thereby improving the cooling efficiency several tens-fold to several hundreds-fold; therefore, it is extremely advantageous from an economical standpoint. As a result, it becomes possible to apply a superconducting wire material to an instrument to which it could not be conventionally applied from an economical standpoint, and the use and the market of superconducting instruments are expected to be expanded to a large extent.
The crystal system of RE-123 superconductors (particularly Y-123 superconductors with the molar ratio of Y:Ba:Cu=1:2:3) is an orthorhombic crystal. Therefore, in order to exert the characteristics of the material in conducting characteristics, it is require to align not only the CuO planes of the crystal but also the in-plane crystal orientations. This is because only small misalignment of orientations generates bicrystal grain boundary and decreases the conducting characteristics.
A production process for fabricating a wire material while improving the in-plane orientation of the crystal in the above-mentioned Y-123 superconductor and aligning the directional property in the plane has the same constitution as that of a production process for a thin film. That is, by forming an intermediated layer in which the in-plane orientation and the directional property are improved on a tape-form metal substrate and using the crystal lattice of this intermediate layer as a template, the in-plane orientation and the directional property of the crystal in the Y-123 superconductor can be improved.
Further, the Jc of a superconductor depends on the crystallinity and the surface smoothness of an intermediate layer, and it has been found that its characteristics vary sensitively according to the condition of the substrate to a large extent.
As a technique for producing a biaxially textured metal substrate in which an in-plane aligned intermediate layer is formed on the above-mentioned tape-form metal substrate, SOE (surface-oxidation epitaxy) method, ISD (inclined substrate deposition) method, IBAD (ion beam assisted deposition) method and RABiTS (rolling assisted biaxially textured substrate) method are known, and many reports in which Y-123 superconducting wire material with a Jc of more than 106 A/cm2 is obtained by forming an intermediated layer in which the in-plane orientation and the directional property are improved on a non-textured or textured metal tape.
Among these, in the formation of an intermediate layer by the IBAD or RABiTS method, a vacuum process by a vapor phase method such as PLD (pulse laser deposition) method is used. In the IBAD method, a combination of Hastelloy/YSZ/Y2O3 is generally used as a biaxially textured metal substrate, on the other hand, in the RABiTS method, a combination of Ni/CeO2/YSZ/CEO2 or the like is generally used, and they have an advantage in that a dense and smooth intermediate layer film can be obtained (see, for example, Non-Patent Document 1).
As for materials of the above-mentioned intermediate layer, various investigations have been conducted, in particular, a CeO2 intermediated layer has a good matching with the crystal lattice of a YBCO layer (Y—Ba—Cu—O superconducting layer) and has a small reactivity with the YBCO layer, therefore, it is known to be one of the most superior intermediate layers, and lots of results are reported.
As described above, in the case where a YBCO layer is formed on a metal substrate, CeO2 has an excellent characteristic as an intermediate layer, however, a CeO2 film has a problem in that a crack is liable to occur due to such as the difference in thermal expansion with the metal substrate, and the film cannot be made thick. For example, in the RABiTS method, YSZ (yttria-stabilized zirconia) is interposed in the intermediate layer on the surface of a textured Ni substrate, and as shown in FIG. 2, it was necessary that a CeO2 film (11), a YSZ layer (12) and a CeO2 film (13) are sequentially formed on the surface of a textured Ni substrate (10), and a YBCO layer (14) is formed thereon.
[Non-Patent Document 1]
A. Goyal et al., Physica C, 357-360 (2001) 903